The present invention is directed to an additive for drilling fluid to avoid borehole problems due to either shale instability or mechanical instability while maintaining a relatively high drilling rate.
Drilling of water-sensitive shales with conventional water-based drilling fluids has been plagued by borehole instability problems related to the swelling and sloughing of shales due to their interaction with water. Such problems include stuck drill pipe, time lost in redrilling shale that has sloughed into the borehole, difficulty in running casing, and the like. Heretofore an avoidance of such problems required the use of a dense viscous aqueous drilling fluid or oil-based drilling fluids. However, these solutions are relatively expensive because of slow drilling rates and/or high drilling mud costs.
The shale destabilizing interaction of water and water-sensitive shale involves a long known and troublesome problem. Osmosis and osmotic pressure forces tend to induce transfer of water from a water-containing drilling fluid to the layers of interstices within the shale, although osmosis is not a total explanation for the problems involved in drilling water-sensitive shale. Garrison, in U.S. Pat. No. 2,165,824, noted that such an interaction involves more than osmosis and, accordingly, a use of salts that provide a high osmotic pressure, i.e., an inhibited aqueous drilling fluid, will not prevent either the heaving or dispersing of the shale. Garrison suggests using a relatively concentrated aqueous solution of an alkali metal silicate. However, it is disadvantageously expensive to maintain an alkali metal silicate as a circulating drilling fluid.
One aspect of the interaction between water and a water-sensitive clayey or shaly earth formation causes a dispersion of colloidal sized particles of clay in a circulating aqueous drilling fluid. Such a dispersion tends to increase the viscosity and density of the drilling fluids. There is a problem of drilling fluid maintenance that may be expensive even in the drilling of clayey earth formations which are not sufficiently active to create a borehole instability problem. Many prior inventors have described combinations of carboxyl group containing polymeric materials that are designed to handle these clay dispersion problems. These solutions relate to clear or low solids drilling fluids or clay-containing drilling muds in which the types and amounts of the polymeric materials are adapted to effect a beneficiation or dispersion of the viscosity-adjusting bentonitic type clays while tending to flocculate and enhance the removal of the other clays, such as those encountered in drilling operations. As is known to those skilled in the art, such clay beneficiating and flocculating aqueous polymer systems are not suited for drilling in water sensitive shales and are generally no more effective than a simple aqueous clay mud with respect to stabilizing a borehole in a water sensitive shale. Such aqueous polymer systems are described in U.S. Pat. Nos.:
______________________________________ 2,775,557 3,070,543 3,081,260 3,323,603 3,558,545 3,511,779 4,201,679 3,985,631 3,032,499 3,039,529 2,948,678 3,116,264 3,197,428 3,336,977 3,366,584 3,654,164 3,743,018 3,254,719 4,268,400 4,340,525 4,384,096 4,137,969 4,472,718 4,008,164 2,911,365 3,948,783 3,040,820 2,718,497 3,070,544 3,338,320 ______________________________________
Scheuerman, in U.S. Pat. No. 3,738,437, discloses a process for drilling water-sensitive shale by first circulating a clear drilling fluid, and then circulating an aqueous solution of partially hydrolyzed polyacrylamide and alkali metal halide, and avoiding borehole instability by circulating a suspension of pre-hydrated bentonite in the solution.
Sperry, in Dutch patent No. 6414645, discloses a cohered inhibited mud system for drilling water-sensitive shale wherein a carboxyl-group containing polymer and water-soluble inorganic salt are dissolved in water in a specific ratio of polymer to salt that is critical. The ratio of polymer to salt must be adapted to cause the surfaces of the clay particle to be slightly swollen, without being dispersed in the water, so that the swelling action seals the clay against further water intake or interaction.